1. Field of the Invention
The present invention relates an image forming apparatus and an image forming method.
2. Description of Related Art
An image forming apparatus such as a copying machine or the like for performing color print has been known. In the color print, four color toners or inks of Y (yellow), M (magenta), C (cyan) and K (black) are used often. For example, color print (image formation) has been performed by using toners of YMCK in an electrophotographic image forming apparatus.
The toner K among the above toners express achromatic colors, however, the achromatic colors can be also expressed by blending the toners of CMY. However, because the achromatic colors based on only CMY is low in the maximum density, thus a large amount of K is used for acromatic colors of middle and high densities. Conversely, the achromatic colors can be reproduced by only a simple color of K. However, the boundary between the chromatic color and the achromatic color becomes remarkable when the color is continuously varied from a chromatic color to an achromatic color and graininess becomes remarkable when the achromatic color is expressed only by the simple color of K causing deterioration of image quality. Therefore, the achromatic colors are expressed by CMYK in general.
Furthermore, in recent years, there is a case where high chroma toners having higher chroma than normal toners are used as toners of CMY to expand the reproducible color gamut. Here, the density characteristics of the normal toner and the high chroma toners will be described with reference to FIGS. 11A and 11B. FIG. 11A shows a toner-based density with respect to a print density for normal CMY toners and a black toner. FIG. 11B shows a toner-based density with respect to a print density for CMY toners of high chroma toners and a black toner.
FIGS. 11A and 11B are color separation curves showing how the CMY toners and the black toner are to be used to reproduce achromatic density when using the normal toners and the high chroma toners. In FIGS. 11A and 11B, an ideal tone curve is represented by a dashed line, a tone curve of CMY toners which are normal toners is represented by a broken line, a tone curve of CMY toners which are high chroma toners is represented by a one-dotted chain line, and a tone curve of a black toner is represented by solid line. The print density shown in the horizontal axis of each graph indicates finished target print density. With respect to the density, the density at a point of 0 is defined as white, and the color approaches to black as the value of density increases and is defined as black at a point at which the value thereof is largest in the graph. The toner-based density shown in the vertical axis indicates the density of toner of each color. The tone curve of the CMY toners is assumed to represent the density (equivalent neutral density) when the C, M, Y toners are blended to form an achromatic color.
The high chroma toners can mainly expand the color gamut of colors having high brightness, however, they have a feature that sufficient densities of achromatic colors cannot be obtained even when the CMY toners are blended. That is, the high chroma toners cannot reproduce colors having low brightness. As compared with the normal CMY toners shown in FIG. 11A, colors of low brightness cannot be reproduced when the CMY toners which are high chroma toners shown in FIG. 11 are used, and thus there is a need to start using K alternatively in low density colors.
Furthermore, with respect to the color print, particularly for the electrophotographic process, there is an image forming apparatus in which the maximum toner use amount is limited (see for example, JP-A-2006-279922).
The reason why the maximum toner use amount is limited is as follows. That is, when an excessively large amount of toner is put on a paper, the toner melted in a fixing step cannot be removed from a fixing roller, and consequently there occurs a phenomenon that the paper having the toner thereon cannot be separated from the fixing roller and thus the paper is to be twined around the fixing roller. Once this phenomenon occurs, the apparatus cannot continue the print operation. Therefore, an operator must stop the image forming apparatus and perform a work of removing the paper from the fixing unit and the like. As described above, because there is a large operational demerit such as reduction in the working efficiency, wasting of materials and the like, such phenomenon is prevented by limiting the maximum toner amount.
The maximum toner amount limitation is implemented by executing a control operation of calculating a total toner amount of C+M+Y+K for each pixel at a signal processing stage in advance, and forcedly reducing signal values of CMY when the total toner amount exceeds a prescribed value in each pixel. With respect to the toner amount, for convenience of calculation, the signal values are calculated on the assumption that the each signal value is proportional to the toner amount, and here, the calculation of converting each signal value to the toner amount is omitted. For example, when it is assumed that original signal values are represented by C, M, Y and K and the signal values obtained by subjecting these original signal values to the maximum toner amount limitation are represented by Ctr, Mtr, Ytr and Ktr, the following calculations shown by the pseudo codes are performed for each pixel, thereby limiting the maximum toner amount.
“Ktr = Ktotal = C+M+Y+KIf [Total > Limit]Over = Total - LimitCtr = C - Over/3Mtr = M - Over/3Ytr = Y - Over/3ElseCtr = CMtr = MYtr = Y”
First, a variable is output as it is because the variable “K” is not subjected to the toner amount limitation. Subsequently, the respective signal values of variables “C”, “M”, and “K” are added and the sum thereof is set as a variable “Total”. When the variable “Total” exceeds a predetermined maximum toner amount “Limit”, the variable “Over” corresponding to the excessive amount is first calculated, and this value is subtracted from each of the variables “C”, “M”, and “Y” equally (or in a ratio based on gray balance) to calculate variables “Ctr”, “Mtr”, and “Ytr”. In the other cases (when the variable “Total” does not exceed the predetermined toner amount “Limit”), Ctr=C, Mtr=M and Ytr=Y are output.
However, a phenomenon that graininess in a low density area is deteriorated when a black toner is used in the low density area occurs in the conventional image forming apparatus using the high chroma toners. The cause for this resides in that the graininess of the black toner as simple substance is worse (larger) than color toners. The graininess is worse as the dynamic range of the density is larger. The black toner has a large dynamic range of the density and thus the graininess is bad.
On the other hand, with respect to the high chroma toners, the dynamic range of the density thereof is small, and thus the graininess thereof is low (good). Conversely, this feature makes the badness of the graininess of the black toner relatively remarkable. This is elicited when flesh color is reproduced.
Furthermore, when the maximum toner amount limitation acts on pixels in the conventional image forming apparatus using high chroma toners, the image density of the pixels is lowered, and thus required high density is not obtained and image information is also lost, and thus the image quality is degraded.